Structural characterization of water and ice in mesoporous SBA-15 silicas: II. The 'almost-filled' case for 86 Å pore diameter

Probing the Behaviour of Fluids Confined in Porous Materials by Neutron Scattering: Applications to CO2 Sequestration and Enhanced Oil and Gas Recovery

The current review presents a discussion on the utility of neutron scattering, with emphasis on neutron total scattering and small-angle neutron scattering (SANS), to explore the structural properties and the phase behaviour of fluids confined in nanopores. The effectiveness of contrast matching SANS on the evaluation of accessibility of porous materials to invading fluids is highlighted too. This review provides also an overview regarding the neutron scattering studies on the structure and the accessibility of greenhouse gases in the complex pore network of geomaterials, with applications to CO2 geological sequestration and enhanced oil and gas recovery.

Anomalous Depletion of Pore-Confined Carbon Dioxide upon Cooling below the Bulk Triple Point: An In Situ Neutron Diffraction Study

The phase behavior of sorbed CO{2} in an ordered mesoporous silica sample (SBA-15) was studied by neutron diffraction. Surprisingly, upon cooling our sample below the bulk critical point, confined CO{2} molecules neither freeze nor remain liquid as expected, but escape from the pores. The phenomenon has additionally been confirmed gravimetrically. The process is reversible and during heating CO{2} refills the pores, albeit with hysteresis. This depletion was for the first time observed in an ordered mesoporous molecular sieve and provides new insight on the phase behavior of nanoconfined fluids.

The Dynamic Response Function χ T(Q,t) of Confined Supercooled Water and its Relation to the Dynamic Crossover Phenomenon

We have made a series of Quasi-Elastic Neutron Scattering (QENS) studies of supercooled water confined in 3-D and 1-D geometries, specifically, interstitial water in aged cement paste (3-D) and water confined in MCM-41-S and Double Wall Nano Tube DWNT (1-D). In addition, we also include the cases of hydration water on protein surface and other biopolymer surfaces (pseudo 2-D). By analyzing the QENS spectra using Relaxing Cage Model (RCM), we are able to extract accurately the self-intermediate scattering function of hydrogen atoms FH(Q,t), at low-Q as a function of temperature T, showing an α-relaxation process at long time. We can then construct the Dynamic Response Function χT(Q,t) = -dFH(Q,t)/dT. χT(Q,t) as a function of t at constant Q shows a single peak at the characteristic α-relaxation time 〈τ〉, the amplitude of which grows as we approach the dynamic crossover temperature TL observed before in each of these geometries. However, the peak height of χT(Q,t) decreases after passing the crossover temperature TL. We make an argument to relate the occurrence of the extremum of the peak height in χT to the existence of the dynamic crossover temperature in each of these cases.

Origin of the enthalpy features of water in 1.8 nm pores of MCM-41 and the large C(p) increase at 210 K

It is shown that exothermic and endothermic features of dH(m)/dt observed on heating rapidly precooled and slowly precooled states of water in 1.8 nm pores of MCM-41 and the unusually large increase in the specific heat in the 210-230 K range [M. Oguni, Y. Kanke, S. Namba, and AIP Conf, Proc. 982, 34 (2008)] are inconsistent with kinetic unfreezing of a disordered solid, or glass softening. The exotherm is attributable to the melt's gradual conversion to distorted icelike structures and the endotherm to the reverse process until their fractional amounts reach a reversible equilibrium on heating. The large increase in C(p,m) with T is attributed to the latent heat, similar to that seen on premelting of fine grain crystals. The available calorimetric data on freezing and melting and the pore-size dependence of the features support this interpretation. The findings also put into question a conclusion from neutron scattering studies that in 1.8 nm pores water undergoes a structural and kinetic transition at approximately 225 K while remaining a liquid.